Caption: Eye anatomy, artwork. At the front of the eye is the cornea, a transparent coating. Behind this is the lens in top diagram only), which is partly covered by the iris. The lens focuses light on the retina at the back of the eye. Light sensitive cells in the retina transmit impulses to the brain via the optic nerve. Behind the retina is the choroid, which contains blood vessels that nourish the back of the eye. The outer layer is the sclera, the white of the eye. The eyeball is filled with a gelatinous substance called vitreous humor. Several muscles hold the eyeball in place and allow it to rotate.